It is very common in battery packs to connect together individual battery cells or strings of battery cells (two or more battery cells in a series configuration wherein the discharge battery and receiving battery are in series and the other, optional batteries in the string of battery cells can be in a series, parallel or combination thereof configuration) in a parallel configuration. Such a parallel configuration makes it possible to obtain higher current or power from the battery pack than would be available from just a single cell or series of cells. When cell strings are connected in parallel, it is possible the different strings could discharge at different rates. Unequal discharge rates can occur, for example, if the electrical connections to one string have higher resistance than the electrical connections to another string or other strings. Another possible cause of unequal discharge rates is the situation in which one (or more) of the cells in one of the strings has higher internal impedance than the other cells. Any situation in which, cell strings that are connected in parallel and discharge at unequal rates can potentially lead to hazardous conditions. A cell in a string that is discharged at a higher rate will reach its end of life more rapidly than cells in other strings that have a lower discharge rate. As a result there can be a risk that those higher discharge rate cells will be (a) discharged deeply or (b) driven into reversal, which can lead to dangerous behavior such as cell venting.
One particularly common situation in which there is notable risk of unequal discharge rates is that presented by very large packs in which it is difficult to maintain an even temperature throughout the pack. If heat is generated in the cells during the discharge, then cell strings that are located in the interior of the pack and which are thus subjected to additional heating from adjacent packs will be warmer than similar cell strings located toward the outside (a.k.a., perimeter) of the pack. Because internal impedance in batteries tends to decrease at higher temperatures, the cell strings that are warmer will have lower impedance and will thus deliver higher current than the cooler strings. Methods exist for controlling the temperature within a battery pack so that the temperature is the same throughout the pack. However, those cooling methods are costly in (a) regard to reduced energy efficiency and increased weight and (b) materials.
Applicant is aware of US published application numbers 2005/0275373 to Huang et al.; 2010/0305770 to Bhowmik et al.; and 2011/0057617 to Finberg et al.; and U.S. Pat. No. 8,026,698 to Scheucher. These references disclose battery packs. Some of those battery packs have a string of battery cells in parallel configurations, switching devices controlled by pulse width modulators, or sensors that measure current or voltages and, as a result of those measurements, the switching devices are turned on or off by the pulse width modulators.
The above-identified references do not disclose two sensors sandwiching an intermediate sense resistor and placed in series with each string of battery cells, in particular in series with the most negative cell in each string of battery cells. There are at least a first string of battery cells and a second/more string of battery cells wherein every string of battery cells are in a parallel configuration. For example, the sensors measure the current being drawn by each string of battery cells and based on the reading of the first string of battery cells and the readings from the other string(s) of battery cells, the duty cycle on the first string of battery cell's pulse width modulator (PWM) switching device adjusts or maintains the current to match the current of the other string(s) of battery cells.
Overall, it would be preferable to use a lighter, less bulky, or less expensive method for balancing the rate of discharge in the separate cell strings that are connected in parallel in a battery pack. The apparatus and method set forth in this application would preferably maintain the same rate of discharge in the separate strings regardless of any temperature differences between the strings.